Advanced Radio Systems, in particular those where information is stored in the amplitude of very short optical pulses, may require conversion by an optical-to-electric component (photodiode) followed by amplification, sampling in a track-and-hold circuit, conversion to digital form in an analog-to-digital converter, and finally feeding to a CPU for information extraction. Existing systems use narrow-band, continuous sine wave modulated light sources to produce a sine wave signal to track-and-hold circuits. The signals are AC coupled and any DC content is lost.
Existing systems are poorly suited for digitizing very short optical pulses (from tenths of picoseconds to tens of picoseconds). Short optical pulses may arise from sampling a high speed signal, thus the light power in each pulse represents the high-speed signal amplitude at various points in time. The difficulty increases rapidly if the pulse repetition frequency (PRF) is increased to rates above ten GHz. Such pulse signals require very broadband amplifiers, DC capable, to maintain fidelity. This type of amplifier is difficult to construct because small DC bias changes at the input of the amplifier can drive the output stages of the amplifier into saturation producing a very non-linear voltage response. The small DC bias changes can be caused by temperature changes, input and output VSWR changes or power supply voltage ripples and noise.
In existing “integrate and dump” circuits, a capacitor is employed as an integrating element of the pulsed photocurrent Ip(t):
      v    ⁡          (      t      )        =            1      C        ⁢                  ∫                  -          ∞                t            ⁢                                    I            p                    ⁡                      (            t            )                          ⁢                  ⅆ          t                    The sharp rise and fall times of the input pulse become smoothed out. The peak amplitude of the smoothed out pulse is proportional to the peak energy in the light pulse. Typically this peak value is sampled in a track-and-hold circuit and is sent to an analog-to-digital converter. After the sample is taken the voltage is quickly shorted to zero in preparation for the next current pulse.
System noise induced by voltage transients created by the sudden shorting of the signal line and a failure to reestablish a DC zero value before the next pulse arrives are major limitations to the “integrate and dump” methodology.
Alternatively, a system may measure the DC offset from the AC coupled amplifier and add the offset value back on the original signal. Matching the instantaneous gain of the circuit for both the AC and DC signals to less than a least significant bit value, timing differences between the AC and DC signal path propagation difference and inter-symbol interference are major limitations to the DC offset methodology.
Consequently, it would be advantageous if an apparatus existed that is suitable for digitizing a signal based on a very narrow voltage pulse with rapid pulse repetition frequency.